7884079643

(16)

(

100 ~ \ 100 /

Rcon — Mco/c

moment to introduce the relation C_uf = 100 - A_fa as the unburned carbon in the fły ash. For this reason, Eq. (4) can be rewritten as

ah£a^p i C_u Q\ U00 -

In this equation, a_fa has been considered equal to 1 (Eq. (8)) taking into account the experimental results and calcu-lations, with a high confidence level.

It can be observed that the above simplifications to calculate for q₄ and q₅, reduce and simplify the number of statistical models to be fitted to obtain the claimed efficiency chart. In the next paragraph, the statistical models fitted to calculate C_uf, CO/(CO + C0₂) and stack temperaturę, T_eg, will be discussed. It should be noted that the last parameter, T_tg, is needed to calculate the exhaust gases enthalpy in Eq. (2).

In order to obtain a simplified model, the influence of the stoichiometric ratio in the furnace («f) and steam power (D_sh) on the unburned carbon in the fly ash, C_Uf, has been plotted. Results of the experimental measurements are depicted in Fig. 2. As can be seen, the unburned carbon increases with increasing steam power and decreases with increasing stoichiometric ratio in the furnace. As steam power is raised at a constant stoichiometric ratio, both the amount of bagasse fed and the combustion air flow ratę increase, sińce the air volumetric flow ratę per unit weight of bagasse is fixed. This, in tum, increases the average gas velocity in the fumace and the fraction of fuel that burns in suspension, rather than in the bed on the stationary grate. The shorter residence time available for combustion in suspension results in an increased unburned carbon carry-over and poorer combustion performance. Therefore, when steam power and bagasse consumption are increased, a higher stoichiometric ratio in the fumace is needed to achieve the same carbon conversion (C_Uf). Taking into account all the experimental data, a statistical model is fitted

C_uf (kgę/kgfa) = 0.854965 + 0.002724D_sh reproducing the experimental dependence of C_Uf on both the stoichiometric ratio at the furnace, a_f, and the steam power, D_sh. In this equation D_sh has units of t/h.

Once <74 is calculated, q₃ and q₂ can also be evaluated. To determine the Chemical carbon heat loss (q₃), the parameter Rcon needs to be calculated by the equation

C^p + 0.375S^P 100

/iico/c being the CO-to-C molecular weight ratio. C^p and S^p are the carbon and sulfur contents of bagasse from ultimate analysis (as received). CO and C0₂ are the carbon monoxide and dioxide concentration in the stack gases, respectiveiy. Substituting Rcon >ⁿ Eq. (3), only the term containing CO and C0₂ remains to be determined from the experiments in order to establish a correlation for the Chemical carbon loss, q₃. Results are presented in Fig. 3, showing a linearly decreasing dependence of this term for increasing stoichiometric ratio, and a weak influence of the steam power. For clarity, in this plot, error bars are only displayed for the measurements corresponding to steam powers of 20 and 50 t/h, respectively. The remaining experimental points represent the mean values for

Fig. 2. Performance of the unburned carbon in fly ash vs. stoichiometric ratio at the exit of the furnace (o₍) for different values of steam power, The depicted solid lines correspond to the different steam powers given by Eq. (17).

a,